The dependence of force and velocity on calcium and length in cardiac muscle segments.

The segment length (SL) dependence of force (F) and light load shortening velocity (VL) was determined for central segments of ferret papillary muscles at different extracellular calcium concentrations. Muscles were maintained at 27 degrees C in a physiological solution which contained in mM: NaCl 140; KCl 5.0; MgSO4 1.0; NaH2PO4 1.0; acetate 20; the pH was 7.4. Calcium concentrations were 1.125, 2.25, 4.5 and 9.0 mM. Total force-segment length relations were determined from both muscle length isometric ( auxotonic ) and segment isometric contractions, and were found to be the same for each contraction mode. The peak force generated at a particular segment length was independent of both the amount of shortening during a contraction and the initial SL. Increasing extracellular Ca2+ shifted the F-SL relation toward greater force and the SL axis intercept toward shorter SL. Maximum peak twitch tension was achieved in 9.0 mM Ca2+. Calcium variations also changed the shape of the total F-SL relation from linear in high Ca2+, to concave in low Ca2+. In order to estimate the active F-SL relations, corrections were made for passive force by two methods. The first assumed that passive force was related to SL, and yielded F-SL relations which were nearly identical to those found for total force. This similarity included the curvature changes observed in different Ca2+ concentrations, a finding which is consistent with the hypothesis that length dependent activation is the cause of force decline at short SL. The second method assumed passive force to be related to muscle length, an approach which would be appropriate if, for example, a connective tissue sheath on the muscle dominated passive behavior. These F-SL curves displayed a plateau above 90% SLmax and appeared to be vertically shifted versions of each other. Such characteristics are consistent with the possible role of an internal load in causing the decline of force at short SL. VL-SL relations were obtained from load clamps to 1 mM, imposed at various times during a segment isometric twitch. The results indicate that 1) VL declines linearly with SL below 90% SLmax and 2) VL-SL relations are shifted to higher velocity and shorter SL axis intercepts by increasing Ca2+. The slopes of the VL-SL relations obtained in different calciums are similar. Although an internal load could explain the calcium dependence of VL, it would not explain the similarity of the slopes of the VL-SL relations found in different calciums .